This invention relates to apparatus and methods for recovering valuable metals. particularly gold using an in line leach reactor. In particular non-limiting aspects, the invention also provides apparatus for dewatering and leach reactors which may be suitable for carrying out the method the invention.
Processes for the recovery of gold from gold bearing feeds have typically involved the use of a cyanidation step to convert the elemental gold into a soluble ionic form. The gold in solution can then be separated from the bulk mineral material which stays in solid form by a simple liquids/solids separation process (e.g. sedimentation or filtration).
The solution containing dissolved gold is then subjected to a gold recovery process such as the carbon-in-pulp process. In this process the gold in solution is adsorbed on an activated carbon substrate in the form of carbon granules and the gold is subsequently recovered.
While such processes have been successful in retrieving a significant proportion of the gold embedded in certain minerals, they suffer from significant disadvantages. For example, if the gold is present as nuggets or flakes larger than microscopic pieces, the cyanidation process, because of the relatively low concentration of the reagents used in the process, will generally not succeed in dissolving the larger pieces of gold. As a result. these larger pieces may be lost with the waste minerals discarded following the cyanidation process.
Furthermore, it is often the case that the minerals associated with gold deposits also include a proportion of native carbon. Unfortunately, this carbon is generally in a form which cannot be readily recovered or separated from the minerals. During the cyanidation process, the native carbon can adsorb a proportion of the gold during the leaching process. Depending upon the level of native carbon present in the mineral, the time taken for the leaching process and the concentration of the leaching reagents (i.e. sodium or potassium hydroxide and sodium or potassium cyanide) the amount of gold lost in this way can be quite significant.
Given the large volumes of mineral material which need to be treated using the cyanidation process, it is not practical to use high concentrations of reagents because of their cost, and also because of the environmental concerns associated with the use of large quantities of dangerous reagents. Thus, one is often faced with a situation where the relatively low concentrations of the reagents require high residence times for leaching. The longer the residence time the greater the proportion of gold which will be adsorbed by the native carbon. Thus the presence of native carbon in the minerals being leached means that a significant proportion, perhaps 25% or even 50% or higher of the gold which goes into solution as a result of the leaching reaction can be adsorbed by the carbon in the mineral and is ultimately lost when the leached mineral solids are discarded.
It is possible to ameliorate this problem to some extent by burning off the carbon in a roasting operation prior to leaching. However, it has been found that roasting, whilst it can drive off a significant proportion of the carbon in the mineral as carbon dioxide, is not totally effective in that a substantial quantity of the carbon can still survive the roasting process and remain in solid form intimately admixed or bound with the mineral. Thus, even after roasting, a significant proportion of the gold may be adsorbed by the remaining native carbon in the mineral during leaching. Furthermore, because the process of roasting is very energy intensive, the economics of the gold recovery process can be significantly worsened. This is particularly in light of the fact that gold deposits generally include only extremely small quantities of gold (of the order of grams per tonne) with the result that a huge amount of energy needs to be expended to roast tonnes of ore only to yield grams of gold.
Thus there is a need for a process and apparatus which avoids the need for a roasting step but which can yield high gold recovery rates not withstanding the fact that the minerals with which the gold is associated may include significant amounts of native carbon and/or pieces of gold of a size which are larger than a microscopic size i.e. large enough to be captured by a screen of 500 microns or even 1000 microns.
It is also desirable that the process and/or apparatus have a broad range of applications such as the recovery of gold from sulphide bearing minerals and concentrates or any other minerals which do not give high recoveries with normal gravity processes. It is even more desirable that the process and apparatus be adaptable to recover other valuable materials such as copper.
The invention provides, apparatus for the separation of a dense valuable material from a feed, including:
concentrator means for forming a concentrator containing the dense valuable material from the feed,
a leach reactor which includes a hollow member with inlet means and outlet means,
supply means for continuously supplying an aqueous leach reagent and the concentrate to the inlet means,
drive means for rotating the hollow member,
flow control means in the hollow member for controlling the rate of flow of the mixture of aqueous leach reagent and concentrate through the hollow member from the inlet means to the outlet means;
a solids/liquids separator for separating pregnant liquor from solids arranged to receive the mixture from the outlet means,
a recovery station for recovering dense valuable material from solution in the pregnant liquor to leave a spent leachate, and
recycle means for recycling the spent leachate to the leach reactor.
The term concentrator includes any form of apparatus for concentrating dense material in a feed or for separating dense material from a feed. Thus it includes conventional jigs or separators such as the xe2x80x9cHarz Jigxe2x80x9d, xe2x80x9cHancock Jigxe2x80x9d or a separator of the type described and claimed in Australian Patent No. 684153 hereinafter referred to as the xe2x80x9cIn Line Pressure Jigxe2x80x9d. It also includes banks of two or more concentrators joined in parallel or series.
An In Line Pressure Jig is a pressurised concentrator which uses an agitated bed to separate dense particulates from a slurry. The slurry flows across the top of the bed with dense particulates from the slurry passing through the bed to be collected in a hutch. The less dense tailings pass over the outer edge of the bed to be discharged via a tailings outlet.
The apparatus may be associated with a conventional gold recovery circuit such as a cyanidation circuit.
Suitably, the apparatus includes at least one concentrator which is an In Line Pressure Jig. The apparatus may include more than one concentrator. Where there is more than one concentrator the concentrators may be in series or in parallel. Most preferably they are in series.
In a preferred form of the invention the apparatus includes two In Line Pressure Jigs in series.
The or each concentrator may include an inlet, an overflow and an outlet. Thus the inlet may be arranged to receive incoming material containing the feed. The incoming material is most suitably mixed with water. The outlet may constitute an outlet for material which has been concentrated by the concentrator. The overflow may be arranged to allow material rejected by the concentrator to flow out of the concentrator.
Means for crushing a feed, such as a gold bearing feed, may be provided in association with the apparatus. The means for crushing may include a grinding mill.
Primary separator means may he associated with the apparatus. The primary separator means may be arranged to receive crushed feed from the means for crushing and to redirect it into a light fines stream, a heavy fines stream and a coarse material stream. Suitably the primary separator means is arranged to redirect the coarse material stream into the means for crushing.
The light fines stream may be directed to a gold removal circuit.
The heavy fines stream may be directed to the or each concentrator.
Suitably the primary separator means includes a cyclone. It may include a plurality of cyclones. Most suitably the cyclones are hydrocyclones.
The heavy fines stream from the primary separator means may be directed to a first concentrator via the inlet thereof. Suitably the overflow of the concentrator may be arranged to direct rejected material from the concentrator to the coarse material stream emanating from the primary separator means.
The outlet of the first concentrator may be directed to the inlet of a second concentrator.
Suitably the concentrate emanating from the outlet of the second concentrator is directed to the leach reactor.
In a particularly preferred form of the invention a dewatering station may be provided to dewater concentrate prior to being fed to the leach reactor. The dewatering station may include a container having a conical base, a valve for metering the overflow of dewatered solids material from an outlet at the bottom of said base and weighing means for measuring the weight of material in said dewatering station, said valve being responsive to measurements of said weighing means to control the rate of outflow of dewatered solids from the outlet of said dewatering station. Thus, in a further aspect the invention provides a dewatering station along the lines of that described herein above.
Suitably the leach reactor is arranged to receive concentrate continuously or intermittently for continuous or intermittent leaching of the concentrate. Most suitably the reactor is in the form of a cylinder closed off at each end. It may include drive means for rotating the reactor. It may include flow control means for controlling flow through the leach reactor. Most preferably it includes a plurality of baffles for controlling the flew of leach material through the leach reactor. Most suitably there are two baffles. The baffles may separate the reactor into three or more zones. One or more openings will be provided in each of the baffles to allow communication between the zones. The openings are most suitably provided in proximity to the wall of the leach reactor to control the flow of leach material therethrough as the reactor rotates. Openings on adjacent baffles are most suitably provided on diametrically opposites sides of the cylinder.
An inlet may be provided at one end of the leach reactor and an outlet at the other end. Most suitably the inlet is provided in line with the axis of the cylinder. Similarly the outlet may be provided at the other end in line with the axis of the cylinder. Most suitably, the inlet is of smaller size than the outlet in order to provide a gradient down which the leach material travels as it moves through the leach reactor.
A second dewatering station may be provided to receive leach material from the leach reactor. It suitably includes an inclined linear action dewatering screen. The second dewatering station may be arranged to provide a solids stream and a pregnant liquor stream. The solids stream may be recycled to the first concentrator. The overflow from a second concentrator may also be recycled to the first concentrator via the inlet.
The pregnant liquor stream may be directed to a gold recovery facility. The gold recovery facility may include an electrowinning station. It may also include a settling tank for settling of any solids in the pregnant liquor prior to electrowinning.
Recycling means may be provided to recycle spent liquor from the electrowinning facility to the settling storage tank. The recycling means may also be arranged from the overflow liquid from the settling storage tank to the inlet of the first concentrator.
In a further aspect the invention provides a method for the separation of gold from a feed including the steps of:
(i) crushing the feed;
(ii) concentrating a mixture of the crushed feed and water to form a concentrate stream containing gold wherein at least 80% of the particles in the concentrate stream have a particle size less than 2,000 microns;
(iii) dewatering the concentrate stream;
(vi) leaching the dewatered concentrate stream continuously in a rotating leach reactor with aqueous reagent to form a pregnant liquor;
(vii) controlling the residence time of the dewatered concentrate in the reactor by controlling the rate of rotation of the leach reactor, and
(viii) recovering gold from the pregnant liquor.
The leachant liquid may be a mixture of a sodium or potassium hydroxide and sodium or potassium cyanide. The concentration of the cyanide is preferably at least 0.5% by weight of cyanide in the leachant/solid mix. Most suitably the cyanide concentration is at least 1.5%.
Preferably the rotation rate of the leach reactor is such as to produce a peripheral speed of rotation of at least 3 metres per minute, more preferably at least 8 metres per minute. The residence time of the leach material in the reactor is preferably less than 10 hours, most preferably less than two hours.
Oxygen may be introduced into the leachant mixture in the leach reactor to facilitate leaching. The oxygen may be obtained from an electrowinning facility for recovering gold from the leachant liquid. Most suitably the oxygen is added by recycled spent leachant liquid after electrowinning to the leach reactor. Oxygen may also be added through bubbling air and/or oxygen into the leach mix.
The leach reaction my be carried out at relatively low temperatures i.e. below 50xc2x0 C., more preferably below 40xc2x0 C. and most preferably at ambient temperature.
Most suitably at least 80% of the concentrate fed to the reactor will have a particle size less than 2000 microns, more preferably less than 1000 microns.
Most suitably the residence time for leaching is adjusted so that at least 70% of gold, more preferably 85% and most preferably at least 90% or even 95% is taken into solution.
Preferably gold recovery from the leachant is by way of electrowinning, or by carbon adsorption or by zinc precipitation processes.
The invention is particularly suitable for recovering gold sulphide and free gold from sulphide and free gold concentrates especially when the gold containing materials will not give high recoveries using normal gravity based processes. Where sulphide bearing concentrates are involved high recoveries may not be readily achievable using low level cyanide leaching conditions. However, more intense conditions such as higher temperatures and/or more concentrated leachant liquid and/or higher leach residence times can increase overall recovery particularly in instances where particles of valuable minerals or metals are partially locked in to other less valuable particles.
Whilst a major application of the invention is the recovery of gold is to be understood that the invention is also applicable to other valuable minerals, such as copper bearing minerals.